Generally, optical rotary encoders used to measure the rotational frequency of a rotating shaft, the distance traveled by a sliding body, or other physical quantity are classified into two categories: light reflective type and light transmissive type. A rotary encoder of light reflective type is shown in FIGS. 1 and 2, where a disk 1 is mounted on a rotating shaft (not shown). The fringe of the disk 1 is provided with reflective areas 2 equally spaced apart circumferentially. Non-reflective areas 3 are formed between the successive reflective areas 2. Disposed close to the fringe of the disk 1 are device-mounting members 4 (only one is shown) each of which is provided with holes 5 and 6. A light-emitting device 7 such as light-emitting diode is fitted in each hole 5, while a light-receiving device 8 is fitted in each hole 6. When the reflective areas 2 on the disk 1 are brought into opposition to some pair of the holes 5 and 6, the light emitted by the corresponding light-emitting device 7 is reflected by the reflective areas 2 and fall on the corresponding light-receiving device 8.
In the rotary encoder constructed as described above, the disk 1 and the device mounting members 4 are mounted within one casing, and the mounting members 4 are disposed at regular intervals near the fringe of the disk 1. Therefore, it has been impossible to make the whole assembly thin. Further, it has been difficult to mount the components.